1. Field of the Invention
The invention relates to a process for the direct synthesis of methylchlorosilanes using chloromethane having a reduced oxygen content.
2. Background Art
Processes for preparing methylchlorosilanes by reaction of silicon with chloromethane in the Müller-Rochow direct synthesis in the presence of suitable catalysts and catalyst combinations are already known. This is described, for example, in CATALYZED DIRECT REACTIONS OF SILICON; K. M. Lewis, D. G. Rethwisch; Elsevier 1993, pages 1 to 66.
In the direct synthesis of methylchlorosilanes, metallic silicon is reacted with chloromethane in the presence of various catalysts, optionally with promoters, with the target product being dimethyldichlorosilane. The mixture of silicon, catalysts and promoters is referred to as contact composition. As over 2,000,000 metric tons per annum of dimethyldichlorosilane are produced worldwide at present, small improvements in the production process such as an increase in the dimethyldichlorosilane selectivity, an increase in the specific space-time yield of dimethyldichlorosilane or an increase in the specific silicon yield therefore have a great economic effect.
The adverse effect of oxygen on the direct synthesis is summarized briefly in a literature review in “Silicon for the Direct Process to Methylchlorosilanes”; thesis, Harry Mortem Rong; University of Trondheim/Norway, Institute for Inorganic Chemistry, 1992 on page 53 in a literature review. Even relatively small amounts of oxygen in the chloromethane permanently reduce the reactivity of the direct synthesis; in addition, the selectivity is also adversely affected by higher amounts of oxygen. On pages 116-117 of that thesis, the adverse effect of oxygen on reactivity and selectivity is described in the context of laboratory experiments. 10 ppm is indicated as a guide value for typical oxygen contents in commercially available, pure chloromethane.
In CATALYZED DIRECT REACTIONS OF SILICON, op. cit., pages 10-11, a value of 99.5-99.8% is given as a typical chloromethane purity for the direct synthesis; this reference does not discuss oxygen as an impurity in any detail. However, those skilled in the art recognize that the removal of oxygen in the ppm range which, for example, can be introduced in traces with the raw materials, is difficult and costly.
EP 893448 A describes the use of boron in the contact composition of the direct synthesis to increase the proportion of dimethyldichlorosilane.